That's true... I've heard people say that if two surfaces were perfectly flat and perfectly smooth, then thermal compound would be unnecessary, but since a processor's IHS is neither flat nor smooth (unless you lap it yourself), you're always need thermal compound. How much you need probably depends more on how smooth a surface is than how flat it is (to a degree), because a rough surface has much, much more surface area to cover than a smooth one. As long as the surface is relatively flat (or slightly concave, to make up for the curve of the IHS), then the smoothness is much more important. Remember, the more thermal compound you have to use, the more resistance to heat transfer there will be.